Multiple-bit pulse code modulation (PCM) is a widely used method of converting an analog audio signal into digital form for storage on a digital medium, such as a compact disk (CD). Generally, an analog-to-digital converter (ADC) samples the amplitude of the input analog signal at a given sampling rate (frequency) Fs to produce a stream of multiple-bit samples representing the amplitude value of the analog signal at each sample time. In standard CDs, the digital data are typically recorded with 16-bit samples and a 44.1 kHz sample rate; however, newer audio formats may use up to 24-bit samples and up to 192 kHz sample rates.
During playback, a digital-to-analog converter (DAC) subsystem converts the PCM samples back to an analog waveform. Delta-sigma DACs, based on a noise-shaping delta-sigma modulator and a following DAC output stage, have found widespread use because delta-sigma DACs generally provide high noise attenuation in the signal band and are relatively inexpensive to manufacture. Typically, prior to conversion, the PCM data are passed through an interpolation filter to increase the sample rate. Using oversampling, the delta-sigma modulator spreads the quantization noise power across the oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, the delta sigma modulator performs noise shaping by acting as a lowpass filter to the input signal and a highpass filter to the noise; most of the quantization noise power is thereby shifted out of the signal band. The noise-shaped modulator output signal is ultimately converted to analog form by the output DAC stage, such as a switched-capacitor or current steering DAC.
A more recently developed digital recording system, known as “Super Audio CD” or “SACD”, records the single-bit audio data stream output by a high-order, single-bit, delta-sigma modulator on a digital storage medium, similar to a DVD-type disk. This format is also commonly referred to as a Direct Stream Digital (DSD) format. In theory, a DSD bit-stream may be supplied directly to a delta-sigma DAC for conversion to an analog audio signal during playback without requiring any digital processing.
Two bit streaming modes, or protocols, have been specified for sending a DSD bit-stream to another device, such as a DAC. In the first or normal mode, a device sending the DSD bit-streams drives the data lines according to the value of the data bits being sent. A bit clock is provided to indicate when the data bits are valid. A receiving device may store the data bits on each rising edge of the bit clock.
The second mode, which is the phase modulation mode, is similar to the normal mode in that the sending device drives the data bits onto the data lines, but the data is stored on a falling edge of the bit clock signal. Furthermore, in the phase modulation mode, each data bit is sent as a pair of bits comprising the actual data bit followed by an inverted data bit. Thus, a ‘0’ data bit is sent as a ‘0’ followed by a ‘1’, and a ‘1’ bit is sent as a ‘1’ followed by a ‘0’. A phase clock is provided to distinguish a data bit from its inverse. When the phase clock is low, the data bit is on the data line, whereas the inverted data bit is present when the phase clock is high.
The two modes of data transmission are further classified according to whether the receiving device provides the bit clock signal. When the receiving device is operating in slave mode the sending device provides the bit clock. On the other hand, the receiving device provides the bit clock when it is configured to operate in a master mode.
Transmitting the DSD bit streams using the phase modulation mode provides redundancy in the bit stream, which may provide some degree of robustness. However, the phase modulation mode requires two clock lines, which may be undesirable. For example, a design for an integrated circuit device may be constrained as to the total number of pins available and may be unable to accommodate two pins for the required clock signals. The need for two clock signals also increases the complexity of circuit boards and connectors and may cause problems with radiated electromagnetic interference.
Accordingly, it would be desirable to provide methods and apparatus for receiving phase modulated DSD bit streams in slave mode using only a single clock signal.